1. Field of the Invention
The present invention relates to a method of producing a disazo pigment, a disazo pigment obtained by the method, and a printing ink obtained therefrom. More particularly, the present invention relates to a method of producing a disazo pigment that can produce printing ink excellent in transparency and flowability, and excellent in flushing property and flushing drain coloration resistance when ink formation is subsequently conducted under non-dry condition, or can produce printing ink excellent in dispersibility when ink formation is subsequently conducted under dry condition, a disazo pigment produced by the method, and printing ink thereof.
2. Description of the Prior Art
Among printing industries, there have been, in recent years, very strong demands for high-speed printing to increase productivity. At present, a printing machine which is able to print around 4 meters per second is prevailing, and the demands for high speed printing are still more and more increasing. Accordingly, as a coloring material, high color strength and flowability are required for printing ink. Further, with the recent progress in graphical printing techniques, printing inks having high transparency are also asked in view of multi-color printing technique. Thus, in the recent enhanced and mass-productive printing industry, need of printing ink excellent even in flowability and transparency in addition to basic properties such as dispersibility is extremely high, and it is really one of key technologies to decide further development in the near future of printing industries.
In contrast with such a recent circumstance of printing industries, offset inks in general process publication have conventionally been obtained by a method in which an aqueous wet cake of a pigment is phase-converted by flushing with a vehicle for ink or a solvent to make ink, or a method in which a pigment obtained by drying an aqueous wet cake is dispersed by strong shearing force in a vehicle for ink.
However, in the former method, stable quality can not be easily obtained because of large influences on quality such as opacification and decrease in strength and the like due to the property lacking in stability of an aqueous wet cake itself of a pigment and to heat applied for a long period of time by poor flushing property. Further, there is a problem of coloration by mixing of a small amount of a disazo pigment component into a drain discharged in flushing.
Consequently, the latter method that can be conducted via a process less influential adversely on such qualities has so far been conducted, primarily. However, ink prepared from a dried material of an aqueous wet cake turns out to be remarkably inferior as a final ink product, in transparency, flowability and color strength as compared with ink prepared from an aqueous wet cake not dried. In the case of disazo pigments, such inferior qualities of ink prepared from a dried substance in comparison with those from a non-dried one are due to the remarkably affected crystal growth of pigment particles, exclusively, which is promoted by the heat applied during the drying process.
In the production of disazo pigments, methods based on simultaneous coupling of two components have conventionally been known. For example, Japanese Patent Application Published (JP-B) No. 55-10630 discloses a method of mixed coupling of acetoacetanilide and a second coupling component having a polar group substituted on its phenyl group. Japanese Patent Application Laid-Open (JP-A) No. 1-110578 discloses a method of coupling a mixture of two or more compounds selected from acetoacetanilide, acetoaceto-o-anisidide, acetoaceto-o-toluidide, acetoaceto-o-chloroanilide, acetoaceto-2,4-xylidide and acetoaceto-2,5-dimethoxy-4-chloroanilide. However, in any of these methods, qualities of ink made from a dried substance of a produced disazo pigment are remarkably inferior in view of transparency, flowability and color strength to those of ink made from a wet cake.
Continuous investigations have been made also on improvement of qualities of ink made from a wet cake. For example, Japanese Patent Application Published (JP-B) No. 45-11026 discloses a method of mixing a disazo pigment with a sulfonic acid compound thereof. Japanese Patent Application Published (JP-B) No. 55-49087 discloses a method using a mixture, as a coupling component, of a polar coupling component having a carboxyl group and/or sulfonic acid group and a non-polar coupling component. Japanese Patent Application Laid-Open (JP-A) Nos. 63-72762 and 63-178169 both disclose an asymmetric disazo compound comprising a polar coupling component and a non-polar coupling component.
Further, recently, Japanese Patent Official Gazette No. 2682749 discloses a method of producing a disazo pigment using, as a coupling component, a mixture of 2 to 40 mol % of acetoacetic acid-m-xylidide, 97.9 to 60 mol % of acetoacetic acid-o-toluidide and 0.1 to 10 mol % of 2-acetoacetaminobenzoic acid. However, it has been known that these coupling components show solubility in water, consequently, flushing is extremely delayed, resulting in growth of crystals owing to heat history of a long period of time, change of hue, and opacification.
Japanese Patent Official Gazette No. 2943996 (filed as an International Patent Application PCT/JP97/00547, Publication No.WO97/31067) discloses a method of coupling a non-polar coupling component and a tetrazo component of benzidines. However, in this method, which uses a single coupling component, basic problems present at a microscopic level such as crystal growth are not solved, and there is no other evaluation than the evaluation that the resulting ink's performance is not sufficient yet.
Additionally, there is also known a method in which the surface of a pigment composition is coated with rosins and a vehicle for printing ink to impart various resistances and suitability to the pigment composition. However, this method does not improve basic abilities of the pigment composition before coating, and the coating material does not have so high effect as to compensate low abilities of the pigment composition itself. For example, Japanese Patent Application Laid-Open (JP-A) No. 1-146962 discloses a method of surface-treating an organic pigment using a dispersing aid composed of a resin, rosin, aluminum resinate, aluminum rosinate or a mixture thereof. However, the amount of this dispersing aid used is as extremely high as 50 to 100 mol % based on the organic pigment, namely, dilution is effected by the dispersing aid and increase in dispersing ability is not admitted from the standpoint of a usual pigment composition, though the dispersing aid contributes to improvement in color developing property from the standpoint of ability per coloring matter equivalent.